Use of Partially Saponified Vinly Ester Polymers in Hot-Melt Adhesive Compositions

ABSTRACT

Hot melt adhesives with good flow characteristics, resistance to gelling at elevated temperature, strong adhesion, and yet which can be easily removed from substrates with water, are formulated with one or more partially saponified vinyl ester homo- or copolymers having a molecular weight below 70,000 Da, a degree of hydrolysis of from 62 to 86 mol %, the adhesives being free of mannitol and anionic surfactants.

Use of partially saponified vinyl ester polymers in hot-melt adhesivecompositions

The invention relates to the use of partially saponified vinyl esterhomopolymers and copolymers in hot-melt adhesive compositions. Hot-meltadhesives (hotmelts) are used in the packaging sector, among others, asadhesives: for example, as adhesives for paper, paperboard, cardboard,and wood. Common hot-melt adhesives are based on ethylene-ethyl acrylatecopolymers, polyamides, and ethylene-vinyl acetate copolymers. In trainwith the increasing recycling of packaging materials there is a need forthe adhesives to be readily detachable from the packaging materials.Ideally they ought to be amenable to removal by washing with water. Theabovementioned standard hotmelts do not meet this condition.

JP-A 2001-220563 discloses a water-soluble or water-dispersible hot-meltadhesive composition which comprises polyvinyl alcohol having a degreeof polymerization of ≦350 and a degree of hydrolysis of ≦80 mol %,mannitol, and polypropylene glycol. A disadvantage there is the tendencyto gel under long-term temperature exposure, owing to the highviscosity. JP-A 62-043477 describes a hot-melt adhesive based on apolyvinyl alcohol having a degree of polymerization of preferably 100 to1500 and a degree of hydrolysis of 20 to 95 mol %, the vinyl alcoholunits being arranged in blocks. JP-A 2003-003140 describes hot-meltadhesives comprising a polyvinyl alcohol (degree of hydrolysis <60 mol%), an anionic emulsifier, and plasticizer. Owing to the anionicemulsifier the water resistance of the bond is unsatisfactory. Adisadvantage of these hot-melt adhesives is their low bond strength,excessively high price, excessively high melt viscosity, and theassociated tendency to gel.

It was an object of the invention to develop a hot-melt adhesivecomposition which exhibits good fluidity, leads to stable bonds, but isreadily removable with water from the material to which sticking wasintended.

The invention provides for the use of partially saponified vinyl esterhomopolymers and copolymers in hot-melt adhesive compositions,characterized in that the vinyl ester homopolymers and copolymers have amolecular weight Mw<70000 and the partially saponified vinyl esterhomopolymers and copolymers have a degree of hydrolysis of 62 to 86 mol%, and the hot-melt adhesive composition contains no mannitol.

Suitable partially saponified vinyl ester homopolymers and copolymersare partially saponified vinyl ester polymers having a degree ofhydrolysis of 62 to 86 mol %, preferably 65 to 75 mol %, most preferably70 mol %. The weight-average molecular weight Mw is <70000, preferably5000 to 30000 (determined by means of gel permeation chromatography),the molecular weight being determined prior to the saponification andthe molecular weight Mw therefore referring to the as yet unsaponifiedvinyl ester homopolymer or copolymer. The hot-melt adhesive compositioncontains no anionic emulsifiers. In the partially saponified vinyl esterhomopolymers and copolymers the vinyl alcohol units are randomlydistributed.

Suitable vinyl esters are vinyl esters of unbranched or branchedcarboxylic acids having 1 to 18 carbon atoms. Preferred vinyl esters arevinyl acetate, 1-methylvinyl acetate, vinyl propionate, vinyl butyrate,vinyl 2-ethylhexanoate, vinyl laurate, vinyl pivalate, and vinyl estersof a-branched monocarboxylic acids having 5 to 13 carbon atoms, examplesbeing VeoVa9^(R) or VeoVa10^(R) (tradenames of Resolution PerformanceProducts), and also mixtures thereof. Vinyl acetate is particularlypreferred.

Besides the vinyl ester units it is also possible where appropriate forone or more monomers to be copolymerized from the group encompassingmethacrylic esters and acrylic esters of alcohols having 1 to 15 carbonatoms, olefins, dienes, vinylaromatics, ethylenically unsaturatedmonocarboxylic and dicarboxylic acids, and vinyl halides. Suitablemonomers from the group of the esters of acrylic acid or methacrylicacid are methyl acrylate, methyl methacrylate, n-, iso-, and tert-butylacrylate, 2-ethylhexyl acrylate, and norbornyl acrylate. Suitable dienesare 1,3-butadiene and isoprene. Examples of polymerizable olefins areethene and propene. Vinyl-aromatics which can be copolymerized arestyrene and vinyltoluene. Examples of suitable monocarboxylic anddicarboxylic acids are acrylic acid, methacrylic acid, crotonic acid,and fumaric acid. From the group of the vinyl halides it is common touse vinyl chloride. Most preference is given to ethylene, crotonic acid,and vinyl chloride. The fraction of these comonomers is calculated suchthat the fraction of vinyl ester monomer is >50 mol % in the vinyl esterpolymer.

Maximum preference is given to partially saponified vinyl acetatehomopolymers, partially saponified vinyl acetate-vinyl lauratecopolymers, vinyl acetate-crotonic acid copolymers, and mixturesthereof.

The partially saponified vinyl ester homopolymers and copolymers can beprepared in a known way from the corresponding vinyl ester polymers bymeans of saponification (hydrolysis). The vinyl ester polymers areprepared in a known way preferably by bulk polymerization, suspensionpolymerization or by polymerization in organic solvents, with particularpreference in alcoholic solution. The polymerization is carried outunder reflux at a temperature of 55° C. to 100° C. and initiatedfree-radically by addition of commonplace initiators.

The saponification of the vinyl ester polymers takes place in a mannerknown per se by for example the belt or extruder method or in a stirredtank, in the alkaline or acidic range with addition of acid or base.Preferably the solid vinyl ester resin is taken up in alcohol, methanolfor example, to set a solids content of 15% to 70% by weight. Thehydrolysis is preferably carried out in the basic range, by addition ofNaOH, KOH or NaOCH₃, for example. The base is used generally in anamount of 1 to 5 mol % per mole of ester units. The hydrolysis iscarried out at temperatures from 30° C. to 70° C. After the end of thehydrolysis the solvent is removed by distillation. Alternatively thepolyvinyl alcohol can be obtained as an aqueous solution by successiveaddition of water while the solvent is distilled off.

The hot-melt adhesive composition may further comprise 0 to 50% byweight, preferably 15% to 50% by weight, of sorbitol, based in each caseon the total weight of the hot-melt adhesive composition. This additionreduces the melt viscosity and enhances the flexibility of the adhesivebond and the adhesion to different substrates.

To reduce the surface tension and the viscosity of the melt it ispossible further to admix the hot-melt adhesive composition with waxes,such as hydrocarbon waxes, polyethylene waxes, and polyamide waxes, forexample. Preference is given to polyamide waxes such asethylenebisstearamide wax. The amount used is preferably 0.1% to 15% byweight, based on the total weight of the hot-melt adhesive composition.

Suitable additives for improving the compatibility of the individualcomponents of the hot-melt adhesive composition are fatty acid esterssuch as glycerol monostearate and glycerol distearate, which whereappropriate are used in a weight ratio of 0.1:1 to 1:1 with respect tothe wax fraction of the hot-melt adhesive composition.

Where appropriate it is also possible for water-soluble plasticizers tobe included, such as glycerol esters of rosin, methyl esters of rosin,pentaerythritol esters of rosin, and polyalkylene glycols, preferablypolypropylene glycol. The amounts in the hot-melt adhesive compositionare 0 to 10% by weight, preferably 1% to 5% by weight, based on theoverall weight of the hot-melt adhesive composition.

Examples of further additives include antioxidants for increasing thegelling stability, based on corresponding phosphites, stericallyhindered phenols, lactones or mixtures thereof. Antioxidants of thiskind are available commercially and are employed where appropriate in anamount of up to 2% by weight, based on the overall weight of thehot-melt adhesive composition.

The hot-melt adhesive composition is suitable for use as adhesive forpaper, paperboard, cardboard, wood, and plastic, especially as a labeladhesive for PET containers. The hot-melt adhesive composition isdistinguished over the prior art by virtue of the fact that the adhesiveis water-soluble and yet remains thermoplastically processible, and theadhesive is readily amenable to removal by washing when substratesbonded using it are recycled.

The examples which follow serve for further illustration of theinvention:

The formulas specified in Table 1 were tested:

Solid resin: partially saponified polyvinyl acetate with degree ofhydrolysis =70 mol % and molecular weight Mw (PVAC)=10000

PPG 400: polypropylene glycol with OH number of 400

Licolub: ethylenebisstearamide wax

Tegomuls: glycerol monostearate TABLE 1 Example C1 2 3 4 5 6 7 Solidresin [g] 60 96 75 70.3 66.0 87.5 81.0 Sorbitol [g] 21 19.7 18.5Mannitol [g] 36 PPG 400 [g] 4.0 4.0 4.0 3.8 3.5 4.7 4.3 Licolub [g] 5.08.0 6.2 9.8 Tegomuls [g] 1.3 4.0 1.6 4.9

The specified formulas were used to conduct the following test, whoseresults are summarized in Table 2:

Determination of Melt Viscosity [Pas]:

The melt viscosity was determined by measuring the hot-melt adhesivesusing a high-temperature rheometer from Bohlin and plotting the rheologycurves in the temperature range 100° C. to 180° C. (gap distance 500 μm,frequency 1 Hz, Def 0.05, temperature ramp 5° C./min, oscillatingmeasurement).

Determination of Water Solubility [s]:

The water-solubility was determined by sealing 2 cotton strips (1 cm×4cm) with the corresponding formulation at 180° C. The sealed bond wasimmersed in a waterbath at room temperature and loaded with a weight of10 g. A measurement was made of the time until the bond had undergonecomplete parting. TABLE 2 Example C1 2 3 4 5 6 7 Viscosity at 1410 2610612 1000 942 333 622 120° C. [Pas] H₂O solubility [s] 245 107 366 170214 250 280

Discussion of Results:

In comparison to the prior art formulation the inventive formulationscombine generally much-improved flow behavior with adhesive films thatexhibit comparable water-solubility.

1-10. (canceled)
 11. A process for adhesively bonding surfaces,comprising applying to at least one surface a molten hot melt adhesive,comprising: at least one partially saponified vinyl ester homopolymer orcopolymer having a molecular weight Mw<70,000; a degree of hydrolysis of62 to 86 mol %; and a random distribution of vinyl alcohol moieties,wherein the hot-melt adhesive composition is free of mannitol andanionic surfactants; and contacting said surfaces to be bonded with saidmolten adhesive therebetween, and allowing said molten adhesive tosolidify.
 12. The process of claim 11, wherein the degree of hydrolysisis 65 to 75 mol %.
 13. The process of claim 11, wherein the molecularweight Mw is 5000 to 30,000.
 14. The process of claim 11, wherein atleast one partially saponified vinyl ester homopolymer or copolymer isselected from the group consisting of partially saponified vinyl acetatehomopolymers, partially saponified vinyl acetate-vinyl lauratecopolymers, and partially saponified vinyl acetate-crotonic acidcopolymers.
 15. The process of claim 11, wherein the hot-melt adhesivecomposition further comprises sorbitol.
 16. The process of claim 11,wherein the hot-melt adhesive composition further comprises at least onewax.
 17. The process of claim 11, wherein the hot-melt adhesivecomposition further comprises at least one fatty acid ester.
 18. Theprocess of claim 11, wherein the hot-melt adhesive composition furthercomprises at least one water-soluble plasticizer.
 19. The process ofclaim 11, wherein the hot-melt adhesive composition further comprises atleast one antioxidant.
 20. The process of claim 11, wherein at least onesurface is a surface of paper, paperboard, cardboard, wood, or plastic.21. A hot melt adhesive composition suitable for use in the process ofclaim 11, comprising: at least one partially saponified vinyl esterhomopolymer or copolymer having a molecular weight Mw<70,000; a degreeof hydrolysis of 62 to 86 mol %; and a random distribution of vinylalcohol moieties, wherein the hot-melt adhesive composition is free ofmannitol and anionic surfactants.
 22. The composition of claim 21,wherein the degree of hydrolysis is 65 to 75 mol %.
 23. The compositionof claim 21, wherein the molecular weight Mw is 5000 to 30,000.
 24. Thecomposition of claim 21, wherein at least one partially saponified vinylester homopolymer or copolymer is selected from the group consisting ofpartially saponified vinyl acetate homopolymers, partially saponifiedvinyl acetate-vinyl laurate copolymers, and partially saponified vinylacetate-crotonic acid copolymers.
 25. The composition of claim 21,further comprising sorbitol.
 26. The composition of claim 21, furthercomprising at least one wax.
 27. The composition of claim 21, furthercomprising at least one fatty acid ester.
 28. The composition of claim21, further comprising at least one water-soluble plasticizer.
 29. Thecomposition of claim 21, further comprising at least one antioxidant.30. The composition of claim 21, wherein the degree of hydrolysis is 65to 75 mol %; the molecular weight is from 5000 to 30,000; the partiallysaponified vinyl ester polymer is a vinyl acetate polymer; and furthercomprising at least two further ingredients selected from the groupconsisting of sorbitol, wax, fatty acid ester, water solubleplasticizer, and antioxidant.